1. Field of the Invention
The present invention relates to a honeycomb heater and a catalytic converter wherein a honeycomb structure is coated with a specific catalyst composition.
2. Description of the Related Art
Honeycomb heaters of the above-described type can be employed as heaters for domestic use, such as hot air heaters, or as industrial heaters, such as preheaters used for control of automobile exhaust emission. The abovedescribed catalytic converters can be used in automobile exhaust emission control.
Catalytic converters for use in automobile exhaust gas control must have a predetermined temperature or above when operated so as to cause its catalyst to undergo catalytic action. Hence, the catalyst must be heated when the temperature thereof is not sufficiently high, i.e., at the beginning of running of a vehicle.
Such techniques for heating the catalyst have been proposed in, for example, Japanese Utility Model Laid-Open No. 67609/1988. This disclosed technique is a catalytic converter comprised of a metal monolithic catalyst disposed upstream of and adjacent to a main ceramic monolithic catalyst. The metal monolithic catalyst comprises an electrically conductive metal substrate with alumina coated thereon.
Furthermore, in Japanese Patent Laid-Open Nos. 156545/1988 and 185451/1988, catalysts for purifying exhaust gas of harmful components such as hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x) have been proposed.
In Japanese Patent Laid-Open No. 156545/1988, for the purpose of obtaining a three-way catalyst for automotive exhaust gas having high initial activity and durability, it is disclosed that a honeycomb substrate having unitary (monolithic) structure is coated with a catalyst composition which comprises zirconia loaded with a platinum-group metal, a refractory inorganic oxide such as activated alumina, and a rare earth element oxide such as a cerium oxide.
In Japanese Patent Laid-Open No. 185451/1988, for the same purpose as aforesaid Japanese Patent Laid-Open No. 156545/1988, a purification catalyst for exhaust gas which comprises a honeycomb substrate with a catalyst composition coated thereon is disclosed, the catalyst composition containing a refractory inorganic oxide in a form of particles such as alumina and zirconia whose average particle diameter ranges 0.5-20 .mu.m; and supporting 5-30% platinum by weight and 1-20% rhodium by weight.
Also, in Japanese Patent Laid-Open Nos. 156545/1988 and 185451/1988, it is described that a metal monolith substrate such as an alloy of Fe-Cr-Al is employed as a honeycomb substrate.
Japanese Utility Model Laid-Open No. 67609/1988, however, does not disclose any adequate catalyst composition specifically. However, it is important that a metal monolithic catalyst which is electrically conductive disposed upstream of the main monolithic catalyst have adequate light-off performance at low temperatures thermal resistance and durability against poisonous substances, such as Pb and P.
Both the catalysts for purifying exhaust gas disclosed in Japanese Patent Laid-Open Nos. 156545/1988 and 185451/1988 were not developed for use with a heater; therefore, when they are coated on a heaters, durability and the like of the heater and the coating needs to be developed, due to high operating temperatures thereof.